modify docs overview url (facebookarchive#1802)

Summary:
Summary

I fixed the broken link in the README.

Test Plan

I tested the change by clicking on the link to make sure that it works now.
Pull Request resolved: facebookarchive#1802

Reviewed By: flarnie

Differential Revision: D9347916

Pulled By: flarnie

fbshipit-source-id: a38dd7ec8d6baa73d694eabf4b7362cab8ca8d23

Author: lockround

README.md

@@ -15,7 +15,7 @@ with [immutable-js](https://facebook.github.io/immutable-js/), offering
 an API with functional state updates and aggressively leveraging data persistence
 for scalable memory usage.
 
-[Learn how to use Draft.js in your own project.](http://draftjs.org/docs/overview.html)
+[Learn how to use Draft.js in your own project.](https://draftjs.org/docs/getting-started.html)
 
 ## API Notice